1. Field of the Invention
The present invention relates to a spring fixing or securing part, and more particularly, to a spring standoff having an open central portion for improving the efficiency of the centering operation of a reciprocating rod displacer during an assembly process.
2. Description of the Related Art
Generally, a variety of reciprocating devices, including but not limited to free-piston machines, are often used in a heat regeneration type of refrigerator, including but not limited to Stirling coolers, Gifford-McMahon refrigerators, and the like.
A conventional free-piston machine is described in U.S. Pat. No. 6,293,184, which issued to Unger on Sep. 25, 2001, the contents of which are expressly incorporated by reference in its entirety. Additionally, hereinafter, the structure and operation of a conventional typical free piston machine is described in FIG. 1, which shows a sectional view of a typical free-piston machine. FIG. 2 is a perspective view of the conventional spring standoff.
The free-piston machine includes a sealing container 10, a cylinder 20 installed in the inside the sealing container 10 for containing a gas therein, a piston 22 mounted in the inside the cylinder 20, a displacer housing 30 provided on one side of the cylinder 20, a displacer 32 movably installed at the inside the displacer housing 30, for compressing and expanding  the gas, a regenerator 40 for absorbing thermal energy from the gas, and a linear motor 50 for driving the piston 22.
The displacer 32 has a displacer rod 321 on one end. The displacer rod 321 penetrates the piston 22 and is supported by a planar spring 12 on the lower side of the cylinder 20. The planar spring 12 linearly oscillates within its range of elastic deformation. The displacer 32 is configured to include the regenerator 40 therein.
A compression space 30a is provided between the piston 22 and the displacer 32, for compressing the gas by the combined movement of the piston 22 and the displacer 32. An expansion space 30b is provided on the front inner side of a finger tube 14 for expanding the gas.
A reciprocating device in the form of a cooler may perform cryogenic refrigeration, and therefore cannot use a lubricant having a liquid component. Also, since the respective elements of the device are regeneration-hardened and are thus brittle, the moving parts such as the piston 22, the displacer 32 and the like should be coaxially positioned. If the moving parts are not concentrically (or coaxially) positioned, the piston 22 and the displacer 32 may frictionally contact the cylinder 20 and the displacer housing 30. The frictionally contacting portion is thus worn away with use, resulting in the fracture and failure of the product.
In order to solve the aforementioned problem, a gas bearing method has been provided that uses a working gas. In this method, the working gas serves as a lubricant of the moving parts, such as the piston 22 and the displacer 32. Specifically, while the working gas is compressed, the gas bearing method allows a small amount of working gas to be injected, and thus the injection pressure of the working gas prevents the piston 22 and the displacer 32 from frictionally contacting the cylinder 20 and the displacer housing 30.
However, according to the aforementioned gas bearing lubrication method, since the  pressure of the injection gas is very low, it is very important to precisely align the concentrically arranged moving parts during the assembly process. In particularly, unlike the piston which is centered by the magnetic field of the linear motor 50, the displacer 32 is moved only by the interaction of the working gas and the planar spring 12 and receives a weak supporting force from the gas bearing, thus it is difficult to maintain a precise concentric state.
Due to the aforementioned difficulty, in the conventional manufacturing process of a linear reciprocating device, the centering process for aligning the center of the displacer 32 with the center of the displacer housing 30 is performed after the installation of the displacer 32.
The known centering process is performed in the following sequence. First, the displacer rod 321 is coupled with the planar spring 12. Afterwards, a protruded end of the displacer rod 321 penetrating the planar spring 12 is moved in the x-axis direction and the y-axis direction (which are both perpendicular to the axial direction of the displacer rod 321) such that the displacer 30 is aligned with the center line of the displacer housing 30. In particular, the displacer rod 321 is fixed after penetrating the piston 22 and the planar spring 12.
After the centering process, the planar spring 12 is coupled with a spring fixing part 11 (also referred to as a “spring standoff”) inside of the sealing container 10, thereby completing the installation operation of the displacer 32.
The conventional centering process using the protruded end of the displacer rod 321 must be performed after the displacer rod 321 is coupled with the planar spring 12. Thus, a jig (known in the art) performing a centering operation must move a considerable amount, since the end of the displacer rod is located a considerable distance from the “origin” from  which the “moment arm” (displacer rod 321) extends, thereby potentially reducing the accuracy of the centering process. Additionally, since the displacer rod 321 often protrudes beyond the spring 12 by only a small amount, it is often difficult to apply the jig. As a result, the precision of the process of centering the displacer 32 is lowered and the manufacturing efficiency of the device is reduced.
In addition, after completion of the centering process, the planar spring 12 must be coupled with the spring standoff 11. Hence, the control of the position of the displacer rod 321 is allowed only when the screw holes 11a of the planar spring 12 and the holes of the spring standoff 11 are not registered in line with each other to some degree (i.e., a degree of aligning freedom is required). As a result, since the freedom movement of the displacer rod 321 is very low, it is difficult to efficiently align the concentric positioning of the reciprocating parts.